Expanding tropics will play greater global role, report predicts

By 2050, half of the world’s population will reside in the tropics—the relatively warm belt that girdles the globe—according to State of the Tropics, a hefty report released today. Rapid population growth, coupled with economic growth, means that the region’s influence will grow in coming decades, the authors of the 500-page tome predict. At the same time, tropical conditions are expanding poleward as a result of climate change, but at a slower rate than previously believed.

“The tropical population is expected to exceed that of the rest of the world in the late 2030s, confirming just how crucial the Tropics are to the world’s future,” said Sandra Harding, project convener and vice chancellor of Australia’s James Cook University, in a statement. “We must rethink the world’s priorities on aid, development, research and education.”

The result of a 3-year collaboration between 12 prominent tropical research institutions, State of the Tropics grew out of an effort to acknowledge the region as an environmental and geopolitical entity in its own right. Geographers define the tropics as the belt that is centered on Earth’s equator, between the Tropic of Cancer and the Tropic of Capricorn (each 23.5° of latitude off the equator). Although tropical regions vary considerably, they are “typically warm and experience little seasonal change in daily temperatures.” These geographic and environmental commonalities play a key part of shaping human societies in the region, which is currently home to about 40% of the world’s population, the authors add.

The project, initially launched in 2011, aimed to answer one overarching question: Is life in the tropics is improving? To find out, the researchers analyzed environmental, social, and economic indicators collected over 6 decades. It finds that the region has made “extraordinary progress” in many areas. For example, there’s been a 14% increase in the proportion of the population with access to safe drinking water, and the number of protected areas is increasing. The tropics also have outperformed the rest of the world in economic growth over the past 30 years; it now represents approximately 18.7% of global economic activity, up from 14.5% in 1980.

Some challenges remain. Investment in research and development, for example, has increased modestly, but tropical nations still invest less than the rest of the world. “As a proportion of GDP the Rest of the World invests almost four times as much in R&D than the Tropics but, given the difference in GDP levels, in expenditure terms the gap is even larger,” concludes the report. Despite the lower investments, however, academic publishing is on the rise. Although tropical nations account for only 5% of scientific and technical journal articles published worldwide since 1990, growth is expanding rapidly—publishing output for the tropics rose at more than double the rate than in the rest of the world from 1990 to 2009.

"The breadth and depth of this report are going to make it a really important document to make decisions moving forward," says Emilio M. Bruna, professor of tropical ecology and Latin American studies at the University of Florida in Gainesville and editor-in-chief of the journal BioTropica.

The report also includes a new analysis of how climate change will affect the region. More than 30 studies confirm that the tropics are expanding poleward as Earth warms, the report notes. While a 2009 estimate calculated that the tropical zone was expanding poleward at a rate of 222 to 533 kilometers every 25 years, the new report estimates that the expansion is occurring more slowly —between 138 and 277 kilometers per 25 years.

Many plant and animal species are moving poleward in an attempt to stay within their preferred environmental conditions, the authors write. However, organisms may not be able to keep pace with the changing conditions, they predict; many species will be able to shift their ranges by just about 42 kilometers every 25 years—lagging about 100 kilometers behind potential climate shifts. As a result, some species could experience population declines or go extinct.

As the tropics creep poleward, the so-called subtropical dry zone could begin to squeeze adjacent, wetter temperate zones, the report warns. The temperate zone, which includes heavily populated areas, now experiences a Mediterranean climate with often wet winters. As drier conditions take hold, however, there could be serious implications for water and agricultural resources.

An expanded tropical zone also poses health and safety issues, the authors warn. As regions become more suitable to insects such as mosquitoes and ticks, the prevalence of insect-borne diseases such as malaria, dengue, and Lyme disease could rise, posing a burden to human health and the economy. Poleward expansion of the tropics also appears to be linked to an expansion in the range of tropical cyclones, potentially bringing weather events to regions which previously have not encountered them.

Bruna says he sees a lot to be concerned about in the report, both from a biodiversity and socioeconomic perspective. “While we’ve made incredible advances in some areas, I think the risks for the tropics that are highlighted in this report are things that we really need to be concerned about, and which I’m not necessarily optimistic that we have well-developed solutions for.”